Image sensors are semiconductor devices which convert an optical image into an electrical signal. Image sensors may be classified as a change coupled device (CCD) image sensor and a complementary metal oxide silicon (CMOS) image sensor (CIS). A CMOS image sensor includes a photodiode and a MOS transistor in each of unit pixels, and thus, sequentially detects electrical signals of the respective unit pixels in a switching mode, thereby displaying an image. As a design rule in the CMOS image sensor has been gradually decreased, the size of the unit pixels is decreased, and thus, a fill factor may be decreased.